The invention pertains to an apparatus for forming a single face web of corrugated paperboard. More particularly, the invention relates to a corrugating roll assembly comprising a large diameter corrugating roll (i.e. a bonding roll) and a small diameter corrugating roll in which the small diameter roll is resilient so that it is capable of deflection in the vicinity of the corrugating nip in order to cushion impact as the rolls mesh along the corrugating nip.
In the manufacture of corrugated paperboard, a single facer apparatus is used to corrugate the medium web, to apply glue to the flute tips on one face of the corrugated medium web, and to bring a liner web into contact with the glued flute tips of the medium web with the application of sufficient heat and pressure to provide an initial bond. For many years, conventional single facers have typically included a pair of fluted corrugating rolls and a pressure roll, which are aligned so that the axes of all three rolls are generally coplanar. The medium web is fed into a corrugating nip formed by the interengaging corrugating rolls. While the corrugated medium web is still on one of the corrugating rolls, adhesive is applied to the flute tips by a glue roll. The liner web is immediately thereafter brought into contact with the adhesive-coated flute tips.
In the past, the fluted corrugating rolls have typically been generally the same size as each other. More recently, a significantly improved single facer apparatus has been developed in which the corrugating rolls comprise a large diameter bonding roll and a substantially smaller diameter roll, with the ratio of diameters preferably being 3:1 or greater. One such apparatus is disclosed in U.S. Pat. No. 5,628,865, and improvements thereon are described in copending application Ser. No. 08/854,953, filed May 13, 1997 and Ser. No. 09/044,516, filed Mar. 19, 1998, and Ser. No. 09/244,904, filed Feb. 4, 1999, all of which disclosures are incorporated herein by reference. In accordance with these disclosures, the single facer typically includes a backing arrangement for the small diameter corrugating roll. One preferred backing arrangement includes a series of axially adjacent pairs of backing idler rollers, each pair having a backing pressure belt entrained therearound. Each of the pressure belts is positioned to bear directly against the fluted surface of the small diameter corrugating roll on the side of the small corrugating roll opposite the corrugating nip. Each pair of associated idler rolls and pressure belts is mounted on a linear actuator, and can thus engage the small diameter corrugating roll with a selectively adjustable force. The application of force against the small diameter corrugating roll, in turn, applies force along the corrugating nip between the small diameter roll and the large diameter roll. Typically, a force of approximately 100 lbs. per linear inch (e.g. 10,000 lbs. for a 100 inch roll) is desirable for properly fluting a medium web at typical line speeds.
The impact of the flutes on the small diameter corrugating roll against the flutes on the large diameter corrugating roll along the corrugating nip can cause undesirable vibrations that can detriment the quality of corrugation. More specifically, chordal action due to the interengagement of the rolls causes the small diameter roll to move up and down. The center axis of the large diameter roll is analytically stationary, and vibrational energy is transmitted primarily to the small diameter roll and to the belted backing arrangement. It has been found that excessive vibrations of the belted backing arrangements is sometimes evident under certain high-speed operating conditions, especially when the system is operated at or near the natural resonance frequency of the system.
The invention involves the use of a small diameter corrugating roll that is designed to cushion contact at the corrugating nip between the flutes on the small diameter corrugating roll and the flutes on the large diameter corrugating and bonding roll. The cushioning by the small diameter corrugating roll reduces the transmission of vibration impulses to the belted backing arrangement, and thus reduces undesired vibrational movement of the small diameter corrugating roll. Reduction of such vibrational movement, and primarily reduction of radial vibrational movement, improves the quality and consistency of the corrugation. It also reduces noise levels and roll wear rate.
In its preferred form, the small diameter corrugating roll is made to be resilient, e.g., constructed using an inner steel tube or carbon fiber tube having approximately a four inch outside diameter and a xe2x85x9 inch wall thickness. Preferably, the small diameter corrugating roll is a composite roll in which the flutes are made of a sacrificial material such as reinforced phenolic resin as described in the above-incorporated copending U.S. patent application Ser. No. 09/244,904. Such flutes are preferably mounted on the outside surface of the resilient steel or carbon fiber tube with epoxy.
In operation, the resilient tube deflects inward as the flutes on the small diameter roll impact the flutes on the large diameter roll at the corrugating nip. This deflection occurs without causing substantial movement of the center axis of the tube for the small diameter roll. Preferably, the maximum inward deflection of the resilient tube is within the range of {fraction (2/1000)} to {fraction (5/1000)} of an inch for typical corrugating loading conditions. While this amount of deflection may seem relatively small, it significantly reduces the amplitude of vibrations transmitted to the belted backing arrangement. After the deflected region passes through the corrugating nip, it springs outward to its normal position. If the flutes are made of a sacrificial phenolic resin or other similar material, the flutes themselves assist in cushioning the impact, although deflection of the resilient tube accounts for a substantial portion of the cushioning.
It is preferred that the flutes on the small diameter corrugating roll have a different profile than the flutes on the large diameter corrugating roll such that there is a clearance between flute tips on the large diameter bonding roll and the gullets or roots of the flutes on the small diameter corrugating roll. In this manner, the medium web fed to the corrugating nip is pressured against the fluted profile of the large diameter corrugating roll as the medium web passes through the meshed flutes in the corrugating nip. Also, inasmuch as wear does not effect the radial distance of the gullets, this arrangement assures that the small diameter corrugating roll follows the bonding roll more consistently.
Another advantage of designing the small diameter corrugating roll with a relatively thin wall thickness is that the reduced weight of the small diameter roll has been found to significantly change the natural resonance frequency for the system. In fact, using a small diameter roll having a thin wall in accordance with the invention typically causes the natural resonance frequency to shift upward outside of practical operating speeds for producing corrugated paperboard.